UAr was extracted in Colorado gas plants, purified at Fermilab and shipped to
LNGS, during an intense cooperation of many years [1]}. The
\(^{39}\) Ar activity of UAr is a factor \((1.4\pm 0.2) \times 10^3\)  lower than the
atmospheric argon one, corresponding to an activity of \((0.73\pm 0.11)\)  mBq/kg
[2]}.
